Optical pickup apparatus, optical disc apparatus, and tracking control method thereof

ABSTRACT

After making such adjustment that an a main push-pull signal MPP offset can be made zero when a main beam is positioned at the center of a main photoreceptor, using electric signals E and F photoelectrically converted by one of two two-divided side beam photoreceptors and electric signals H and G photo-electrically converted by the other thereof, a side push-pull signal SPP including a constant α is determined for allowing respective (F−αE) and (αH−G) offsets to be made zero and is calculated according to the formula, SPP−(F−αE)+(αH−G). From this SPP signal and the main push-pull signal MPP, a differential push-pull signal DPP is then calculated, thereby allowing stable tracking control to be carried out, even during recording.

TECHNICAL FIELD

The present invention relates to an optical pickup device, an opticaldisc apparatus using the optical pickup device, and a tracking controlmethod for controlling a tracking for the optical disc apparatus.

The present invention more particularly relates to an optical pickupdevice and the like wherein if a differential push-pull signal iscalculated from a main beam and a pair of side beams, which arereflected from the optical disc, as recording information on an opticaldisc and/or reproducing the information from the optical disc, the mainpush-pull signal is adjusted so that an main push-pull offset is madezero when the main beam is positioned at the center of a mainphoto-detection system and a pair of side beams are positioned at bothsides of the main beam with them being symmetrical about the main beam;and then, a constant α is previously set so that offsets of the sidepush-pull signals are respectively made zero, thereby allowing theoffsets of the side push-pull signals to be cancelled even if the sidebeams fail to be set on the center of photo-detection system, and thestable tracking control to be carried out even at the recording.

BACKGROUND ART

Differential push-pull (DPP) method has been known as one of thetracking servo systems used in a drive apparatus for an opticalrecording disc such as CD-R and CD-RW.

According to the DPP method, calculation based on output signals fromeach photoreceptor, which are respectively obtained from the main beamand two side beams, allows a tracking error signal to occur.

Particularly, a beam emitted by a laser diode (light source) 1 formsthree beam spots consisting of a spot generated by zeroth-orderdiffraction light (hereinafter referred to as “main beam 21”) and twospots generated by first-order diffraction light (hereinafter referredto as “side beams 22A, 22B”) on an optical disc 6 through diffractingmeans (a diffraction grating) 2, collimator lens 4, and objective lens5, which are set on a forward course of the emitted beam as shown inFIG. 1.

A center 36 (hereinafter referred to as a “centerline of track”) of eachtrack 26 in the optical disc 6 rotationally driven is irradiated withthe main beam 21 as illustrated in an illustration encircled by dottedlines shown from A direction in FIG. 1. A disc surface between the trackand an advanced adjacent track is irradiated with the side beam 22A,which defines it as an advanced beam. Another disc surface between thetrack and a subsequent adjacent track is irradiated with the side beam22B, which defines it as a subsequent beam. Respective photoreceptors ofphoto-detector 8 receive reflected lights for these beams (according toa theory of tracking).

The photo-detector 8 comprises a main photo-detector (MPD) for receivinga main spot by the main beam 21, side photo-detector (SPD1) forreceiving a side spot from the side beam 22A, and side photo-detector(SPD2) for receiving a side spot from the side beam 22B, as illustratedin an illustration encircled by dotted lines shown from B direction inFIG. 1.

In the photo-detector 8, the main photo-detector (MPD) is divided into 4pieces of elements vertically and horizontally each receiving the mainspot by the main beam 21. Respective side photo-detectors SPD1, SPD2 aredivided into 2 pieces of elements vertically and the side photo-detectorSPD1 receives the side spot by the side beam 22A as well as the sidephoto-detector SPD2 receives the side spot by the side beam 22B.Incidentally, referring to output signals from the respective dividedelements as A, B, C, D, E, F, G, and H, respectively, the calculationbased on these signals allows the tracking servo error signal to occur.

The main push-pull signal (MPP) occurs based on the output signal fromthe MPD. Side push-pull signals (SPP1 and SPP2) occur based on theoutput signals from the respective SPD1 and SPD2. DPP signals areobtained according to a following operational expression:MPP=(A+D)−(B+C)SPP=SPP1+SPP2=(F−E)+(H−G)

$\begin{matrix}{{DPP} = {{MPP} - {K*{SPP}}}} \\{= {\left( {A + D} \right) - \left( {B + C} \right) - {K\left\{ {\left( {F - E} \right) + \left( {H - G} \right)} \right\}}}}\end{matrix}$where K is a constant set for adjusting a light intensity of thediffraction light of zeroth-order, and plus and minus first-order.

The DPP method has such a drawback that offset occurs in the SPP signalwhen the side beams 22A, 22 b are not positioned at their center of theSPD1, SPD2 and others. If so, two signals SPP1, SPP2 have almost thesame amplitude, thereby allowing cancellation of the offset by theaddition of these signals at the reproduction. At the recording,however, they have different amplitudes, thereby failing to cancel theoffset. This causes the tracking servo error signal including theoffset, resulting in some inconvenience that the tracking is unstable,at the worst case of de-tracking thereof, and the less recordingperformance is delivered.

DISCLOSURE OF THE INVENTION

The present invention is applied to an optical pickup apparatus forrecording information on a groove of an optical disc and/or reproducingthe information from the groove by irradiating a main beam on thegroove, irradiating an advanced side beam on the groove and ansubsequent land adjacent thereto. The apparatus comprises a lightsource, diffracting means for diffracting light emitted from the lightsource to form a main beam of zeroth-order light, and a pair of sidebeams of plus and minus first-order lights, the side beams beingpositioned at both sides of the main beam with the zeroth-order lightbeing fitted between the first-order lights, light receiving means forreceiving the main and side beams formed by the diffracting means andreflected by the optical disk, the light receiving means includingfour-divided main beam photoreceptor for receiving the main beam tophotoelectrically convert and a pair of two-divided side beamphotoreceptors for receiving the pair of side beams, each forphotoelectrically converting, first calculating means for calculatingmain push-pull signal MPP according to a formula, MPP=(A+D)−(B+C), whereA, B, C, and D are electric signals photoelectrically converted by thefour-divided main beam photoreceptor included in the light receivingmeans, second calculating means for calculating side push-pull signalSPP according to a formula, SPP=(F−αE)+(αH−G), where E, and F areelectric signals photoelectrically pheteelee4ei4eally converted by oneof the two-divided side beam photoreceptors included in the lightreceiving means, and H, and G are electric signals photoelectricallyconverted by the other two-divided side beam photoreceptors included inthe light receiving means, and third calculating means for calculating adifferential push-pull signal DPP including a constant K according to aformula, DPP=MPP−K*SPP, the differential push-pull signal DPP forming atracking error signal using the main push-pull signal calculated by thefirst calculating means and the side push-pull signals calculated by thesecond calculating means.

The constant α is previously set to adjust to making an MPP offset zerowhen the main beam is positioned at the center of the main photoreceptorand the pair of side beams are positioned at both sides of the main beamwith them being symmetrical about the main beam and to adjust to making(F−αE) and (αH−G) offsets zero.

The optical pickup device according to the present invention allows anSPP signal offset to be cancelled by the calculation when the side beamsare not positioned at the centers of the SPDs. The optical pickup devicefor performing stable tracking control during the recording may beimplemented.

Namely, according to the invention, since side push-pull signal offsetis cancelled with the calculation, stable tracking control may beimplemented even when the side beams have different amounts of light toeach other during the recording and the other. This allows recordingperformance (RF jitter or the like in the recorded disc) to be improved.Further, electrically adjusting the SPP signal makes tolerance forjustification of optics extended and eliminates the need for ajustification mechanism and an adjustment thereof This allows a cost ofmanufacturing the optical pickup device to be cut.

The invention is also applied to an optical disc apparatus forperforming a tracking control on an optical disc based on a differentialpush-pull signal. The optical disc apparatus comprises driving means forrotationally driving the optical disc, and an optical pickup device forrecording information on the optical disc thus rotated by this drivingmeans and/or reproducing the information from the optical disc.

This optical pickup device includes a light source, diffracting meansfor diffracting light emitted from the light source to form a main beamof zeroth-order light, and a pair of side beams of plus and minusfirst-order lights, the side beams being positioned at both sides of themain beam with the zeroth-order light being fitted between thefirst-order lights, light receiving means for receiving the main andside beams formed by the diffracting means and reflected by the opticaldisc, the light receiving means including a four-divided main beamphotoreceptor for receiving and photoelectrically converting the mainbeam and a pair of two-divided side beam photoreceptors for receivingand photoelectrically converting the pair of side beams, firstcalculating means for calculating main push-pull signal MPP according toa formula, MPP=(A+D)−(B+C), where A, B, C, and D are electric signalsphotoelectrically converted by the four-divided main beam photoreceptorincluded in the light receiving means, second calculating means forcalculating side push-pull signal SPP according to a formula,SPP=(F−αE)+(αH−G), where B, and F are electric signals photoelectricallyconverted by one of the two-divided side beam photoreceptors included inthe light receiving means, and H, and G are electric signalsphotoelectrically converted by the other two-divided side beamphotoreceptors included in the light receiving means and thirdcalculating means for calculating a differential push-pull signal DPPincluding a constant K according to a formula, DPP=MPP−K*SPP, thedifferential push-pull signal DPP forming a tracking error signal usingthe main push-pull signal calculated by the first calculating means andthe side push-pull signals calculated by the second calculating means.

The constant α is previously set to adjust to making an MPP offset zerowhen the main beam is positioned at the center of the main photoreceptorand the pair of side beams are positioned at both sides of the main beamwith them being symmetrical about the main beam, and to adjust to making(F−αE) and (αH−G) offsets zero.

The optical disc apparatus according to the present invention allows anSPP signal offset to be cancelled by the calculation when the side beamsare positioned not at the centers of the SPDs because the above opticalpickup device is included in the optical disc apparatus. The opticaldisc apparatus for performing stable tracking control during therecording may be implemented. Thus, this invention provides inexpensiveoptical disc apparatus of which the recording performance can beimproved.

The invention is further applied to a tracking control method forcontrolling a tracking in an optical pickup apparatus for recordinginformation on a groove of an optical disc and/or reproducing theinformation from the groove by irradiating a main beam on the groove,irradiating an advanced side beam on the groove and an advanced landadjacent thereto, and irradiating a subsequent side beam on the grooveand an subsequent land adjacent thereto. This method comprises the stepsof diffracting light emitted from a light source to form a main beam ofzeroth-order light, and a pair of side beams of plus and minusfirst-order lights, the side beams being positioned at both sides of themain beam with the zeroth-order light being fitted between thefirst-order lights, irradiating the main and side beams thus formed onthe optical disc, and four-divisionally receiving and photoelectricallyconverting the main beam reflected by the optical disc as well astwo-divisionally receiving and photoelectrically converting the pair ofside beams, calculating main push-pull signal MPP according to aformula, MPP=(A+D)−(B+C), where A, B, C, and D are electric signalsobtained by photoelectrically converting the main beam, calculating sidepush-pull signal SPP according to a formula, SPP=(F−αE)+(αH−G), where E,F, H, and G are electric signals obtained by photoelectricallyconverting the pair of side beams, calculating a differential push-pullsignal DPP having a constant K according to a formula, DPP=MPP−K*SPP,the differential push-pull signal DPP forming a tracking error signalusing the main push-pull signal and the side push-pull signals.

When performing the calculation, the constant α is previously set byadjusting to making an MPP offset zero when the main beam is positionedat the center of the main photoreceptor system and the pair of sidebeams are positioned at both sides of the main beam with them beingsymmetrical about the main beam, and then, adjusting to making (F−αE)and (αH−G) offsets zero.

The tracking control method for the optical disc apparatus according tothe present invention allows an SPP offset to be cancelled by thecalculation when the side beams of the optical pickup device are notpositioned at the centers of the SPDs. This permits stable trackingcontrol for the optical pickup device to be implemented during therecording.

Further, according to the invention, since no de-tracking occurs evenwhen the side beams have different amounts of light to each other duringthe recording and the like, recording performance (RF jitter or the likein the recorded disc) may be improved. Further, electrically adjustingthe SPP signal makes tolerance for justification of optics extended andeliminates the need for a justification mechanism and an adjustmentthereof. This allows a cost of manufacturing the optical pickup deviceto be cut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating a configuration of anoptical pickup device embodying the invention.

FIG. 2 is a conceptual diagram illustrating a configuration ofphoto-detector and an ideal example of positional relationship betweenmain beam and side beams.

FIG. 3 are diagrams showing waveforms of respective signals in thephoto-detector as shown in FIG. 2: FIG. 3A shows a waveform of MPPsignal; FIG. 3B shows a waveform of SPP1 signal; FIG. 3C shows awaveform of SPP2 signal; and FIG. 3D shows a waveform of DPP signal.

FIG. 4 is a conceptual diagram illustrating a situation that the sidebeams are moved off in the photo-detector.

FIG. 5 are diagrams showing waveforms of respective signals in thephoto-detector as shown in FIG. 4: FIG. 5A shows a waveform of MPPsignal; FIG. 5B shows a waveform of SPP1 signal; FIG. 5C shows awaveform of SPP2 signal; and FIG. 5D shows a waveform of DPP signal.

FIG. 6 is a block diagram showing a configuration of a calculationsystem in the optical pickup device according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has been contrived in the above circumstances byadopting a relative simple way to save the inconvenience and an objectof the present invention is to provide an optical pickup device allowingstable tracking control to be implemented, which is capable of cancelingthe offset during the recording when side beams are not positioned atthe center of each of the SPDs, an optical disc apparatus using such theoptical pickup device, and a tracking control method used for theoptical disc apparatus.

The optical pickup device embodying the present invention will bedescribed more in detail with reference to the attached drawings.

The optical pickup device 10 as shown in FIG. 1 embodies the presentinvention and is a device for recording information on a rotationallydriven optical disc and/or reproducing the information from the opticaldisc.

The optical pickup device 10 comprises a laser diode (light source) 1, adiffraction grating 2, a beam splitter 3, collimator lens 4, objectivelens 5, optical disc 6, a concave lens 7, a photo-detector 8, a frontmonitor diode 9, and a driving means 15. The front monitor diode 9detects a light intensity of the laser diode 1. The driving means 15drives the optical disc 6 rotationally.

The optical pickup device 10 adapts the DPP method as the tracking servosystem. In the apparatus 10, beam light emitted by the laser diode(light source) 1 is divided into one main beam 21 and two side beams 22Aand 22B through diffraction means (the diffraction grating 2). Thecollimator lens converts respective beams to parallel beams so that therespective beams are concentrated on the optical disc 6 via theobjective lens 5.

Return beam light thereof is then concentrated on the photo-detector 8through the concave lens 7 with the main beam 21 being concentrated onmain photo-detector (main beam photoreceptor MPD) and the side beams 22Aand 22B being concentrated on side photo-detectors (side beamphotoreceptors SPD1, SPD2), respectively, as shown in FIG. 2. Among thesignals received by the respective photo-detectors MPD, SPD1, and SPD2,the main beam 21 is used for recording or reproducing the signal and fordetecting servo error signal. The side beams 22A, 22B are used fordetecting the servo error signal.

According to this tracking servo system, as shown in FIG. 2, the opticssuch as the diffraction grating 2, the laser diode 1, and thephoto-detectors MPD, SPD1, and SPD2 are designed so that they allow therespective beams to focus on each of the centers of the photo-detectorsMPD, SPD1, and SPD2. FIGS. 3A through 3D show ideal waveforms of mainpush-pull (MPP) signal and side push-pull signals (SPP1 and SPP2) inthis case. In these drawings, a vertical axis indicates an amplitude ofsignal and a horizontal axis indicates time, t.

It is conceivable that following two factors make the beam moved off thecenter of the photo-detector such as MPD, SPD1 and SPD2. First,positional misalignment in a horizontal direction of FIG. 2 in thephoto-detector 8 or a shifted amount of the objective lens in a trackingdirection is conceived. In this case, the main beam 21 and the sidebeams 22A, 22B are moved off in the same direction by the same amount ofdeviation.

Second, it is conceived that a variety of splitting angle ofzeroth-order diffraction light and first-order diffraction light basedon the positions of the diffraction grating 2 and the laser diode 1causes different distances between the main beam 21 and the side beams22A, 22B on the optical disc 6 and the photo-detector MPD. In this case,the side beams 22A, 22B are moved off in the opposite directions on thephoto-detectors SPD1, SPD2 by the same amount of deviation with the sidebeams being symmetric with respect to the main beam 21, as shown in FIG.4.

FIGS. 5A through 5D respectively show waveforms of MPD, SPP1, SPP2 andDPP signals in this time. In FIGS. 5B through 5D, pedestal levels ofrespective signals are indicated by chain dash lines and chaindouble-dash lines. Estimating that solid line of MPP signal (indicatedas time axis, t) shown in FIG. 5A is set as reference pedestal level,they indicate positional deviation from the reference pedestal level. Anamount of the positional deviation indicates offset. This referencepedestal level is based on a situation where the main beam 21 isirradiated to the centerline of track 36 on the optical disc 6 shown inFIG. 1.

When the side beams 22A, 22B have almost the same amount of light duringreproduction or the like, only a positional adjustment of thephoto-detector 8 is carried out so that the main beam 21 is positionedat the center of the main photo-detector MPD, thereby canceling thedeviation based on the first factor by subtracting SPP signals(SPP1+SPP2) from the MPP signal. The deviation based on the secondfactor is also cancelled by adding SPP1 signal to SPP2 signal, asindicated by solid lines shown in FIGS. 5B and 5C.

Thus, if the side beams 22A, 22B are not positioned at centers of theside photo-detectors as shown in FIG. 4, respective SPP1 and SPP2signals have the same amount of an offset to be cancelled by apredetermined calculation, thereby resulting in the DPP signal indicatedby the solid line of FIG. 5D. Controlling a tracking servo with such theDPP signal prevents the tracking from being unstably carried out.

On the other hand, during the recording, a pit affect subsequent beamsuch as the side beam 22B but does not affect advanced beam such as theside beam 22A. This causes different amount of light in side spots ofthe side beams 22A, 22B, thereby resulting in different amplitudes ofthe SPP1 and SPP2 signals, as shown in FIG. 5B and the dotted line shownin FIG. 5C.

When a deviation occurs according to the first and second factors, theside beams 22A, 22B respectively fail to be positioned at the centers ofthe side photo-detectors SPD1, SPD2. Thus, if the side beams 22A, 22Bhave different amount of light, their amounts of offsets are differentas a case of combination of the solid line shown in FIG. 5B and thedotted line shown in FIG. 5C, thereby resulting in DPP signal asindicated by the dotted line shown in FIG. 5D. Controlling the trackingservo with this DPP signal causes the failure of de-tracking.

Therefore, in order to avoid an influence of positional deviation of thephoto-detector 8, the objective lens 5 is controlled so that an offsetof the MPP signal is made zero with the optical disc 6 rotating, thelaser diode 1 being generated, and only the focus servo being carriedout. Further, under this condition, according to the invention, acalculation system shown in FIG. 6 performs calculations. In thisembodiment, it multiplies output from the detector SPD1 by a constant αso that amplitude of electric signal (for example, E signal) convertedby one of the side photo-detectors SPD1 divided in two in the sidephoto-detector 8 can be the same amplitude of electric signal (forexample, F signal) converted by the other of the side photo-detectorsSPD2 divided in two therein, and then performs the calculation accordingto a formula, SPP signal=SPP1 signal+SPP2 signal. This allows leveladjustmen side photo-detectors, SPD1, SPD2 zero and then the calculationaccording to a formula, α signal*SPP1 signal+SPP2 signal, to obtain SPPsignal.

Referring to FIG. 6, the calculation system comprises adders 111 through113, subtracters 121 through 124, and constant multipliers 131, 132.

First calculation means 11 includes the adders 111, 112, and thesubtracter 121. The first calculation means 11 performs the calculationaccording to a formula, (A+D)−(B+C) to output MPP signal. For example,the adder 111 adds electric signals A and D; the adder 112 adds electricsignals B and C; and the subtracter 121 subtracts electric signal (B+C)from electric signal (A+D).

Second calculation means 12 includes the adder 113, the subtracters 122,123, and the constant multiplier 131. The second calculation means 12receives α, F, E, H, and G and performs the calculation according to aformula, SPP=(F−αE)+(αH−G)=(F−G)+α(H−E), to obtain SPP signal. Forexample, the subtracter 122 subtracts electric signal E from electricsignal H; the subtracter 123 subtracts electric signal G from electricsignal F; the constant multipliers 131 multiplies electric signal (H−E)by the constant α; and the adder 113 adds electric signal (F−G) andelectric signal α(H−E).

Third calculation means 13 includes the subtracter 124 and the constantmultipliers 132. The third calculation means 13 receives SPP and MPPsignals and performs the calculation according to following formula:

$\begin{matrix}{{DPP} = {{MPP} - {K*{SPP}}}} \\{= {\left( {A + D} \right) - \left( {B + C} \right) - {K\left\{ {\left( {F - G} \right) + {\alpha\left( {H - E} \right)}} \right\}}}}\end{matrix}$to obtain DPP signal. For example, the constant multipliers 132multiplies SPP signal by the constant K; and the subtracter 124subtracts K*SPP signal from MPP signal. This allows SPP signal offset tobe cancelled by the calculation. If the side beams 22A, 22B havedifferent amount of light, stable tracking servo is performed.

Although the optical pickup device embodying the invention has beendescribed, this invention is also applied to an optical disc apparatususing such the optical pickup device and a tracking control method usedfor the optical pickup device.

For example, according to the tracking control method for the opticalpickup device, if recording information on the optical disc 6 and/orreproducing the information from the optical disc 6, light emitted fromthe laser diode 1 is first diffracted to form the main beam 21 ofzeroth-order light, and a pair of side beams 22 a, 22B of plus and minusfirst-order lights, which are positioned at both sides of the main beamwith the zeroth-order light being fitted between the first-order lights.

The main beam 21 and the side beams 22A, 22B are irradiated on theoptical disc 6, and the main beam 21 reflected by the optical disc 6 isreceived by four-divisional way with MPD to photoelectrically convert aswell as the pair of side beams 22A, 22B are received by two-divisionalway with SPD1 and SPD2, respectively, to pheteeleetefieallyphotoelectrically convert.

The first calculation means 11 calculates main push-pull signal MPPaccording to a formula, MPP=(A+D)−(B+C), where A, B, C, and D areelectric signals obtained by photoelectorical conversion of the mainbeam 21. The calculation means 12 calculates side push-pull signal SPPaccording to a formula, SPP=(F−αE)+(αH−G), where E, F, H, and G areelectric signals obtained by photoelectorical conversion of the pair ofside beams 22A, 22B. Further, the calculation means 13 calculates thedifferential push-pull signal DPP that is a tracking error signal,having a constant K according to a formula, DPP=MPP−K*SPP, using themain push-pull signal and the side push-pull signals.

At the time when the above calculation is performed, an adjustment iscarried out so as to make an MPP offset zero when the main beam ispositioned at the center of the main photoreceptor system (MPD) and thepair of side beams are positioned at both sides of the main beam withthem being symmetrical about the main beam. The constant α is thenpreviously set so that (F−αE) and (αH−G) offsets can be made zero.

This allows SPP signal offset to be cancelled by the calculation if theside beams 22A, 22B are not positioned at the centers of SPDs, therebypermitting stable tracking control of the optical pickup device 10during the recording.

Industrial Applicability

This present invention is useful when it is most preferably applicableto a digital video record and reproduction apparatus such as Audio andVideo Disc Recorder (A&VDR) and Digital Versatile Disc-Recorder (DVD-R),which is capable of recording digital video data, digital audio data,and the like on an optical disc and reproducing the data from theoptical disc.

1. An optical pickup apparatus for recording information on a groove ofan optical disc and/or reproducing the information from the groove byirradiating a main beam on the groove, irradiating an advanced side beamon the groove and an advanced land adjacent thereto, and irradiating asubsequent side beam on the groove and subsequent land adjacent thereto,said apparatus comprising: a light source; diffracting means fordiffracting light emitted from said light source to form a main beam ofzeroth-order light, and a pair of side beams of plus and minusfirst-order lights, said side beams being positioned at both sides ofthe main beam with the zeroth-order light being fitted between thefirst-order lights; light receiving means for receiving the main andside beams formed by said diffracting means and reflected by the opticaldisc, said light receiving means including a four-divided main beamphotoreceptor for receiving and photoelectrically converting the mainbeam and a pair of two-divided side beam photoreceptors for receivingand photoelectrically converting the pair of side beams; firstcalculating means for calculating main push-pull signal MPP according toa formula, MPP=(A+D)−(B+C), where A, B, C, and D are electric signalsphotoelectrically converted by the four-divided main beam photoreceptorincluded in said light receiving means; second calculating means forcalculating side push-pull signal SPP according to a formula,SPP=(F−αE)+(αH−G), where E, and F are electric signals photoelectricallyconverted by one of the two-divided side beam photoreceptors included insaid light receiving means, and H, and G are electric signalsphotoelectrically converted by the other two-divided side beamphotoreceptors included in said light receiving means; and thirdcalculating means for calculating a differential push-pull signal DPPincluding a constant K according to a formula, DPP=MPP−K*SPP, saiddifferential push-pull signal DPP forming a tracking error signal usingthe main push-pull signal calculated by said first calculating means andthe side push-pull signals calculated by said second calculating means,wherein the constant α is previously set to adjust an MPP offset to zerowhen the main beam is positioned at the center of the four-divided mainbeam photoreceptor and the pair of side beams are positioned at bothsides of the main beam and are symmetrical about the main beam by making(F−αE) and (αH−G) offsets equal to zero.
 2. An optical disc apparatusfor performing a tracking control on an optical disc based on adifferential push-pull signal, said optical disc recording informationon a groove thereof and/or reproducing the information from the grooveby irradiating a main beam on the groove, irradiating an advanced sidebeam on the groove and an advanced land adjacent thereto, andirradiating a subsequent side beam on the groove and an subsequent landadjacent thereto, said optical disc apparatus comprising: a lightsource; diffracting means for diffracting light emitted from said lightsource to form a main beam of zeroth-order light, and a pair of sidebeams of plus and minus first-order lights, said side beams beingpositioned at both sides of the main beam with the zeroth-order lightbeing fitted between the first-order lights; light receiving means forreceiving the main and side beams formed by said diffracting means andreflected by the optical disc, said light receiving means including afour-divided main beam photoreceptor for receiving and photoelectricallyconverting the main beam and a pair of two-divided side beamphotoreceptors for receiving and photoelectrically converting the pairof side beams; first calculating means for calculating main push-pullsignal MPP according to a formula, MPP=(A+D)−(B+C), where A, B, C, and Dare electric signals photoelectrically converted by the four-dividedmain beam photoreceptor included in said light receiving means; secondcalculating means for calculating side push-pull signal SPP according toa formula, SPP=(F−αE)+(αH−G), where B, and F are electric signalsphotoelectrically converted by one of the two-divided side beamphotoreceptors included in said light receiving means, and H, and G areelectric signals photoelectrically converted by the other two-dividedside beam photoreceptors included in said light receiving means; andthird calculating means for calculating a differential push-pull signalDPP including a constant K according to a formula, DPP=MPP−K*SPP, saiddifferential push-pull signal DPP forming a tracking error signal usingthe main push-pull signal calculated by said first calculating means andthe side push-pull signals calculated by said second calculating means,wherein the constant a is previously set to adjust an MPP offset to zerowhen the main beam is positioned at the center of the four-divided mainbeam photoreceptor and the pair of side beams are positioned at bothsides of the main beam and are symmetrical about the main beam by making(F−αE) and (αH−G) offsets eciual to zero.
 3. A tracking control methodfor controlling a tracking in an optical pickup apparatus for recordinginformation on a groove of an optical disc and/or reproducing theinformation from the groove by irradiating a main beam on the groove,irradiating an advanced side beam on the groove and an advanced landadjacent thereto, and irradiating a subsequent side beam on the grooveand a subsequent land adjacent thereto, said method comprising the stepsof: diffracting light emitted from a light source to form a main beam ofzeroth-order light, and a pair of side beams of plus and minusfirst-order lights, said side beams being positioned at both sides ofthe main beam with the zeroth-order light being fitted between thefirst-order lights; irradiating the main and side beams thus formed onthe optical disc, and four-divisionally receiving and photoelectricallyconverting the main beam reflected by the optical disc as well astwo-divisionally receiving and photoelectrically converting the pair ofside beams; calculating main push-pull signal MPP according to aformula, MPP=(A=D)−(B=C), where A, B, C, and D are electric signalsobtained by photoelectrically converting the main beam; calculating sidepush-pull signal SPP according to a formula, SPP=(F−αE)=(αH−G), where E,F, H, and G are electric signals obtained by photoelectricallyconverting the pair of side beams, wherein when calculating adifferential push-pull signal DPP having a constant K according to aformula, DPP=MPP−K*SPP, said differential push-pull signal DPP forms atracking error signal using the main push-pull signal and the sidepush-pull signals, and a constant α is previously set by adjusting anMPP offset to zero when the main beam is positioned at the center of thefour-divided main beam photoreceptor system and the pair of side beamsare positioned at both sides of the main beam and are symmetrical aboutthe main beam making (F−αE) and (αH−G) offsets eciual to zero.